This invention relates to polymer electrolyte fuel cells, and, more, particularly to air-breathing polymer electrolyte fuel cells. This invention was made with government support under Contract No. W-7405-ENG-36 awarded by the U.S. Department of Energy. The government has certain rights in the invention.
Fuel cell systems developed to date have generally been relatively high power, sophisticated and costly systems intended for space and transportation applications. These systems require a number of subsystems to serve such needs as cooling, humidification/water management, and reactant pressurization, all of which must be integrated with one another to attain optimal performance from the assembled fuel cell stack.
An emerging class of polymer electrolyte fuel cells (PEFCs) is designed for low power applications, such as now served by conventional batteries. The PEFC has attractive features of energy density and ease of refueling that make the PEFC attractive for a number of such applications. But a relatively low-cost, unsophisticated fuel cell is needed that is not humidified, cooled, or pressurized. In a simple embodiment, air (oxygen) is supplied to a cathode by diffusion from the fuel cell periphery so that there is no requirement for forced convection through the flow-field to replenish the depleted oxygen. That is, the fuel cell stack is "air-breathing."
It will be appreciated that such simple fuel cell stacks are useful in a number of military, commercial, and consumer applications. The specific energy of the system can be tailored for a particular device with the appropriate choice and size of hydrogen storage medium. For example, some applications might desire the simplicity and relatively high energy density of pressurized hydrogen storage. On the other hand, concerns for consumer safety can be met with the use of a metal hydride container for the storage of the hydrogen fuel.
An appropriate fuel cell for these types of applications is the PEFC. When compared to other types of fuel cells such as phosphoric acid, molten carbonate, or solid oxide, the PEFC possesses substantial advantages because of its combination of low temperature operation, ability to start-up quickly, simplicity, and benign electrolyte.
Air-breathing fuel cell stacks designed to-date are smaller versions of the higher powered pressurized stacks. In most cases, the air cathodes are not manifolded, but consist of plates with large, vertically oriented grooves that allow the ambient air to flow upward through the warm stack by a chimney effect. While such cells are capable of good performance, the performance may be erratic. While the considerable amount of free convection that is incurred by the chimney effect delivers substantial amounts of oxygen, it can also remove substantial amounts of water. Hence, the cells tend to dry out unless special additional precautions are taken, e.g., a water reservoir that must be periodically replenished. The cells may also tend to overheat when run at high power and the chimney effect is disrupted if the cells are tilted, resulting in a loss of power.
Accordingly, one object of the present invention is a PEFC air-breathing stack configuration is provided that is relatively independent of stack orientation.
It is another object of the present invention to minimize water loss and drying of the cell.
Yet another object of the present invention is to minimize over-heating of the cell.
One other object of the present invention is to provide a PEFC stack with a simple geometry and of minimum size and weight.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.